The Lack of BLR in Low Accretion Rate AGN as Evidence of their Origin in the Accretion Disk

In this paper we present evidence suggesting that the absence or presence of Hidden Broad Line Regions (HBLRs) in Seyfert 2 galaxies is regulated by the rate at which matter accretes onto a central supermassive black hole, in units of Eddington rate. We use the intrinsic (i.e. unabsorbed) X-ray luminosities of these sources and their black hole masses (estimated by using the well known relationship between nuclear mass and bulge luminosity in galaxies) to derive the nuclear accretion rate in units of Eddington. We find that virtually all HBLR sources have accretion rate larger than a threshold value of $\dot{m}_{thres} \simeq 10^{-3}$ (in Eddington units), while non-HBLR sources lie at $\dot{m} \ls \dot{m}_{thres}$. These data nicely fit predictions from a model proposed by Nicastro (2000), in which the Broad Line Regions (BLRs) are formed by accretion disk instabilities occurring in proximity of the critical radius at which the disk changes from gas pressure dominated to radiation pressure dominated. This radius diminishes with decreasing accretion rates; for low enough accretion rates (and therefore luminosities), the critical radius becomes smaller than the innermost stable orbit, and BLRs cannot form.